Functionally graded tungsten–copper coatings produced by axial injection shrouded atmospheric plasma spray

This work investigates the fabrication and microstructural evolution of tungsten–copper (W Cu) functionally graded material (FGM) coatings produced using a shrouded axial-injection atmospheric plasma spray (APS) process. The central novelty of this study lies in the use of an inert-gas shrouded axial-injection APS system, which markedly suppresses in-flight oxidation and enables deposition of W-rich layers, overcoming a major limitation of conventional APS for refractory metals and allowing the formation of a continuous W Cu compositional gradient. The resulting architecture exhibits a controlled transition from a Cu-rich region at the substrate interface to a W-rich layer at the coating surface. Microstructural characterisation by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and quantitative XRD revealed phase-pure W and Cu without evidence of intermetallic formation, together with a well-defined spatial gradient in elemental distribution. Through systematic adjustment of stand-off distance, gas composition, and plasma net power, it was observed that elevated net power and reduced stand-off distance improved melting efficiency and splat deformation, leading to significantly enhanced bonding and reduced porosity, particularly in W-rich sections where deposition is most sensitive to thermal input. Porosity analysis demonstrated that coatings produced under higher plasma power exhibited enhanced consolidation, with porosity decreasing from ~2 % to ~0.2 % in W-rich regions of the FGM after sub-solidus heat treatment at 650 °C in an argon atmosphere. Heat treatment further promoted diffusion-assisted healing of crack defects. Microhardness values reflected the compositional gradient, ranging from ~125 HV in Cu-rich zones to ~310 HV in W-rich regions. These findings establish shrouded APS as a viable, oxidation-controlled method for producing graded W Cu coatings and provide the processing foundation for future mechanical, thermal, and functional assessment in high-temperature and plasma-facing applications. • W Cu graded coatings produced via shrouded APS with reduced oxidation. • Layered gradients achieved uniform Cu W transition and lower interface porosity. • Annealing at 650 °C in Argon facilitated crack healing in W-rich layers. • SEM/EDS/XRD confirmed enhanced densification, homogenisation, and phase stability.